That metric is the total number of seconds spent in GC, it will increase over time with every young gc which is expected. Whats interesting is the rate of growth not the fact that its increasing. If graphing tool has option to graph derivative
you should use that instead.

The gc log file is best to share when asking for help with tuning. The top of file has all the computed args it ran with and it gives details on what part of the GC is taking time. I would guess the CPU spike is from full GCs which with
that small heap of a heap is probably from evacuation failures. Reserving more of the heap to be free (-XX:G1ReservePercent=25) can help, along with increasing the amount of heap. 8GB is pretty small for G1, might be better off with CMS.

we are using G1GC and noticing garbage collection taking a while and during that process we are seeing cpu spiking up to 70-80%. can you please let us know. if we have to tune any parameters for that. attaching the cassandra-env file with jam-options.<cassandra-env.txt>
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